1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Discussion of the Background
An image forming apparatus employing a liquid discharging recording system using a recording head that discharges ink droplets are used as, for example, a printer, a facsimile machine, a photocopiers, a plotter and a multiple function machine. The image forming apparatus employing this liquid discharging recording system form, record, print, or print and transfer images by discharging ink droplets from the recording head to a recording medium while in transfer. The image forming apparatus is typified into a serial type image forming apparatus in which the recording head discharges ink droplet to form images while moving in the main scanning direction and a line type image forming apparatus in which images are formed by ink droplets discharged from the recording head without moving the recording head.
In the present invention, the image forming apparatus employing liquid discharging recording system represents a device that forms images by landing ink on a medium such as paper, thread, fiber, cloth, leather, metal, plastic, glass, wood or ceramics. The image formation represents providing a meaningless image such as a pattern to a medium (i.e., simply landing droplets onto a medium), in addition to provision of images having a meaning such as a letter or a drawing. The ink is not limited to ink in a generic sense but includes all the liquids such as recording liquid or fixing disposal liquid that can be used for image formation. Material for use in the recording medium is not limited to paper but includes a transparent sheet, cloth, etc. where ink droplets are attached. Therefore, the recording medium is used as a generic term that includes a recorded medium, a recording sheet (paper), etc.
In the image forming apparatus employing such liquid discharging system, precision of landing the liquid droplets discharged from the recording head is required to improve the quality of formed images. In addition to the structure of the recording head that discharges droplets, the recording medium should be necessary to be precisely transferred to secure good planarity of the recording medium, and the accurate distance between the recording head and the recording medium.
To improve the transfer accuracy and planarity, recording media are stuck to a transfer belt while in transfer. Also, displacement and floating of a recording medium is prevented by keeping a constant distance between the recording medium and the recording head and precisely controlling paper feeding of the recording medium. Thus, jamming due to contact between the recording medium and the recording head or contamination has been prevented.
Some image forming apparatuses include a transfer path which transfers a recording medium to the image formation position of the recording head and reverses the recording medium to change the direction thereof approximately 90° so that the feeding direction of the recording medium from the paper feeder is reversed to the transfer direction of the recording medium at the image formation position.
When a thick and/or stiff recording medium such as a post card is transferred through a transfer path having such a reverse portion (reverse path) and passes through the nip portion of a pair of rollers that are provided on the upstream side of the reverse portion relative to the recording medium transfer direction, such a thick and/or stiff recording medium paper tends to jump. This may cause deterioration of the feeding accuracy and lead to degradation of the quality of images.
There is an image forming apparatus which reduces displacement of a recording medium which causes accidental displacement of droplets due to the mechanical structure by releasing the pinching roller when the rear end of the thick recording medium passes through the nip (contact portion) formed between the transfer roller and the pinch roller.
In addition, a transfer path dedicated to transfer a thick recording medium is provided in some cases.
However, in the case of the image forming apparatus described above having a structure in which the pair of rollers are released when the recording medium passes through the nip portion formed between the transfer roller and the pinch roller at the reverse potion, the mechanism tends to be complicated and a timely release is difficult, thereby causing the image quality unstable. In addition, when a stiff recording medium such as a post card is used, the displacement caused after the pass-through of the pair of rollers increases, which may have a significant adverse impact on images.